Each of the post melting steel treatment systems in use today is well adapted from a technical standpoint to achieve the results which are demanded of it. However, each system is designed, as it must be, to accommodate the maximum demands which can be envisioned for the system and, as this invention has demonstrated, each such system has inherent deficiencies of a technical or economic nature, or both.
The conventional vacuum arc degassing system enables a user to lower oxygen and hydrogen contents of molten steel to low levels by the use of a sub-atmospheric pressure (or vacuum) which may be as low as less than 1 mm Hg if flake-free hydrogen levels in large sections are desired, an alternating current electric arc which is struck directly between the AC electrodes and the molten steel, and inert gas purging. A typical example can be seen from U.S. Pat. Nos. 3,236,635 and 3,501,289 with respect to which the present invention is, in part, a further development Almost invariably, the vacuum in the U.S. Pat. No. 3,501,289 system, which system is known as the vacuum arc degassing system, is generated by a plurality of steam jet ejectors and it requires, in the U.S. at least, licensed boiler tenders to operate. Also, in the vast majority of commercial installations, the inert gas purging is derived from, preferably, one, or at most, two porous bricks, each of which admits from 3-5 cu. ft./min of purging gas to the molten steel. In some instances a tuyere which produces the same stirring characteristics may be substituted for the purging brick.
Such a system is relatively expensive to build since the steam jet ejector system is relatively expensive Further, such a system is relatively costly to operate due to the cost of generating steam and operators licensing requirements. It has, however, gained wide acceptance due to the ability to achieve the desired low gas results, as well as many other now well recognized advantages over prior systems including temperature and chemical homogenization, contact applications and others.
The ladle furnace is essentially a ladle to which a non-airtight arc furnace cover and electrodes have been added together with a gas purging capacity. The ladle furnace, or LF, is thus capable of heating and purging steel and hence has found application as a holding vessel in a continuous casting system. It is possibly the least expensive of all the post melting systems in that a fully functioning unit may be constructed for only about $250,000. The LF, however, has no vacuum capacity and hence the now universally recognized benefits of vacuum treatment cannot be attained. Its functions are therefore largely limited to temperature and chemical homogenization and holding operations, all of which are useful in continuous casting system.
The DH system utilizes a purging gas in the up leg of an elevated treatment chamber and a high vacuum in the treatment chamber to cause untreated molten steel in a lower, atmospherically exposed source vessel, such as a ladle, to flow upwardly into the treatment chamber where it is subjected to the action of the vacuum before flowing back to the source vessel through a down leg which discharges from the treatment chamber. This system invariably includes a multi-stage steam jet ejector system connected to the treatment chamber to generate the high vacuum therein needed to treat the thin layer of steel flowing from the inlet to the outlet.
Although multi-stage steam jet ejector systems are effective in generating absolute vacuum levels of 1 mm Hg, and even 0.5 mm Hg, they have certain undesirable characteristics. First and foremost is the problem of cleaning. A heat of steel fresh from a melting unit gives off large quantities of dirt and dust when subjected to a vacuum, and this dirt and dust lowers the efficiency of the steam ejector system. Cleaning the ejectors is a disagreeable task which causes the system to be shut down for substantial periods of time at rather frequent intervals--weekly, or even oftener in high production shops.
The following characteristics of steam ejectors may be noted as a background comparison for the advantages attainable with the present invention.
(1) Steam is required for operation. Steam requires a boiler which in turn requires maintenance. The heat energy in the steam is lost, largely, and hence, by comparison as will be apparent hereafter, steam is an expensive motive fluid.
(2) A steam ejector system is a wet system, hence steam condensers are required. Since the steam entrains dust and dirt, a sludge is created which is difficult to handle and dispose of and which plugs ejectors, thereby lowering their efficiency.
(3) A minimum vac of 0.5 mm Hg is attainable, although a more realistically attainable level is 1 mm Hg.
(4) Excellent 0 reduction is attainable, although with special processing such as increased purging rates and/or chemical deoxidation even better 0 reduction is possible.
(5) The lowest possible H reduction of all commercially available systems is attainable.
(6) A purging gas rate of from 3-5 cfm per approximately each 50 short ton increment of heat size is usual.
(7) It is quite expensive to purchase and operate because (a) the steam ejectors are quite costly, (b) the boiler is costly, (c) water treatment systems are costly, (d) steam generation costs are approximately 10 times higher than air used as a motive fluid, (e) sludge handling systems are costly as contrasted to dust handling systems, and (f) a large isolating valve is required.
The combination of a ladle furnace and a ladle degasser, either in the form of two vessels or a single vessel with a vacuum cover which does not contain electrodes and a non-vacuum cover which carries electrodes or other heating means, has also come into use. This system has a relatively high initial cost, particularly as it has been offered by ASEA which includes induction stirring and, of necessity, a stainless steel holding vessel. Again, the vacuum system invariably employed is the steam jet ejector system which has the characteristics mentioned above.
The RH system utilizes a stationary holding vessel and a vertically reciprocable treatment chamber vessel in which a vacuum can be applied. By manipulation of the relative vertical positions of the two vessels and/or variations in the degree of vacuum applied, a portion of the total melt is drawn into the upper treatment vessel where it may be treated by vacuum and then returned to the lower vessel. After a number of cycles, the total melt will have been treated. If a vacuum of 1 mm Hg is applied in the treatment chamber vessel, molten steel in the bottom vessel can be raised up to about 5 feet. Again, this system utilizes a steam jet ejector with the characteristics earlier described.
A recent proposal has been the so-called VAX treatment system. This system, though it does not utilize a steam jet ejector system, is capable of substantial improvement in the post melting phase of steel processing utilizing, in essence, the law of partial pressures to lower the content of undesired gases. This system is described in U.S. Pat. No. 4,655,826 which also discloses the use of arc heating, and to which reference is made for a more complete understanding. It is highly desirable, however, that the art have access to a system which achieves all, or substantially all, of the advantages of the steam jet ejector system when used in applications requiring very low absolute pressures, and arc heating, but at a lower equipment and operating cost, and is simpler to operate. This need is met by the use of an air ejector applied to any one of the conventional treating systems, either as a sole source of sub-atmospheric pressure, or as a supplement to an existing sub-atmospheric pressure